metals Article Study of Solidification Cracking Susceptibility during Laser Welding in an Advanced High Strength Automotive Steel Gautam Agarwal 1,* , He Gao 1,† , Murugaiyan Amirthalingam 2 and Marcel Hermans 1 1 Department of Materials Science and Engineering, Faculty of 3mE, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands; [email protected] (H.G.); [email protected] (M.H.) 2 Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India; [email protected] * Correspondence: [email protected]; Tel.: +31-152-786-036 † Current address: Tata Steel, P.O. Box 10000, 1970 CA IJmuiden, The Netherlands. Received: 10 August 2018; Accepted: 24 August 2018; Published: 28 August 2018 Abstract: Susceptibility to weld solidification cracking in transformation-induced plasticity steel sheets was studied using a modified standard hot cracking test used in the automotive industry. To vary the amount of self-restraint, bead-on-plate laser welding was carried out on a single-sided clamped specimen at increasing distances from the free edge. Solidification cracking was observed when welding was carried out close to the free edge. With increasing amount of restraint, the crack length showed a decreasing trend, and at a certain distance, no cracking was observed. With the aid of a finite element-based model, dynamic thermal and mechanical conditions that prevail along the transverse direction of the mushy zone are used to explain the cracking susceptibility obtained experimentally. The results indicate that the transverse strain close to the fusion boundary can be used as a criterion to predict the cracking behavior. The outcome of the study shows that optimum processing parameters can be used to weld steels closer to the free edge without solidification cracking. Keywords: weld solidification cracking; thermal mechanical FE-modeling; AHSS; automotive; laser welding 1. Introduction Solidification cracking, known as hot tearing in casting, is one of the prominent defects in welding, casting and/or additive manufacturing. Mitigating the occurrence of solidification cracking is important to achieve good weldability. For more than five decades, solidification cracking in welding/casting of various alloys has been studied extensively [1–4]. Recently, increasing environmental concern has motivated the automotive industry to develop thinner gauge steels with high strength, ductility and toughness. The use of lightweight steel in the automotive body reduces the average fuel consumption. Reduced fuel consumption leads to lower CO2 emission, thereby making the vehicle environmentally greener. For this purpose, a class of steels, called Advanced High Strength Steels (AHSS), is being continuously put into use. AHSS differs from mild steels in terms of chemical composition and microstructure. The higher alloying content in AHSS renders it susceptible to solidification cracking during welding. During weld metal solidification, grains grow perpendicular to the fusion boundary since the temperature gradient is steepest, and accordingly, the heat extraction is maximized. The solidifying dendrites meet at the weld centerline. Upon solidification, the solidifying weld metal shrinks due to solidification shrinkage and thermal contraction. As solidification progresses, the solid in the mushy zone begins to form a rigid continuous Metals 2018, 8, 673; doi:10.3390/met8090673 www.mdpi.com/journal/metals Metals 2018, 8, 673 2 of 15 network, i.e., tensile strain is induced by the surrounding material. If the deformation exceeds a certain threshold, separation of the dendrites at the grain boundary can occur. At the terminal stage of solidification, such an opening cannot be compensated by the remaining liquid due to both low permeability and a high solid fraction [5,6]. As a result, solidification cracking occurs. Solidification temperature range, segregation of impurity elements, morphology of solidifying grains, interdendritic liquid feeding and dendrite coherency are some of the important metallurgical aspects affecting the solidification cracking tendency [7]. In a recent study [8], it was reported that impurity elements like phosphorus lead to narrow liquid channels that are difficult to fill with the remaining liquid at the last stage of solidification. The shrinkage-brittleness theory proposed by Pumphrey et al. [1] is based on the concept of coherency temperature. At the coherency temperature, the solid in the mushy region begins to form a rigid continuous network. The theory says that cracking occurs when there is some extent of coherency in the mushy region, i.e., below the coherency temperature. Pellini [2] proposed a strain-based theory for hot cracking in castings. This theory has been extended to weld solidification cracking and states that cracking occurs due to the rupture of liquid films that persist until the last stage of solidification. Building upon the work of Pellini, Prokhorov [3] considered the mushy zone as a single entity and defined the ductility of a material as solidification occurs. Later on, Zacharia [9] used the stress-based approach to describe the thermo-mechanical behavior in the weld pool vicinity. Eskin et al. [10] in their review paper elaborated existing hot tearing theories and models. These models are based on the existence of a critical stress, critical strain or critical strain rate criterion, which leads to cracking. Recent physical models from Rappaz et al. [11] and Kou [12] indicate that in the presence of local deformation, insufficient liquid feeding in the interdendritic or intergranular region results in cracking. Matsuda et al. [13] and Coniglio et al. [4], based on the experiments, concluded that the presence of a critical strain rate is responsible for cracking. However, Katgerman [14] in his mathematical model considered stresses and insufficient feeding in the vulnerable temperature range to be the cause of hot cracking. Experiments focusing on determining the physical mechanism that leads to the separation of grains at the microscopic level under welding conditions are still lacking. Some automotive industries have reported solidification cracking in certain AHSS grades during laser welding [15]. For instance, Larsson [16] reported that solidification cracking can occur during laser welding of B-pillar made of Ultra High Strength Steels (UHSS) used in the Volvo XC60 car body (Figure1). In order to test the resistance to solidification cracking in thin sheets, VDEh(German Steel Institute) has developed a self-restraint hot cracking test [17]. In this test, bead-on-plate laser welding is conducted on single-sided clamped rectangular steel sheets with dimensions of 90 × 45 mm2. The starting position is set at a distance of 3 mm from the free edge with the welding direction inclined 7° to the edge. Crack length is used to assess the hot cracking susceptibility of various alloys. The test is of practical importance for overlap and flange welding geometries in order to minimize the flange width. A typical car body has 40 m of weld flanges that are welded using resistance spot welding [18]. Resistance spot welding requires 16 mm of the flange width to fit the electrode system on either side [19]. The flange width can be minimized by using laser welding (Figure2) and can lead to an overall weight reduction of up to 30–40 kg [18]. The distance from the free edge at which the welding is carried out is considered important for cracking behavior in terms of the response of the material to the amount of restraint. In this work, a different approach was adopted compared to the standard hot cracking test. Different laser welding experiments were conducted at various starting positions from the free edge at an inclination of zero degrees, i.e., parallel to the free edge. This approach was followed, • to maintain a constant restraint with respect to distance from the free edge during welding, • to understand the effect of self-restraint on susceptibility to cracking, • to identify the critical condition as a function of welding distance from the free edge in which no crack occurs, Metals 2018, 8, 673 3 of 15 • to develop an FE-based model to understand the process and delineate the critical factor(s) that may be responsible for the cracking behavior. Figure 1. Solidification cracking in the B-pillar of a Volvo XC60 car body. With permission from [16]. Load Flange Flange width Load width (a) Resistance spot welding (b) Laser welding Figure 2. A schematic showing the flange width required in the case of: (a) resistance spot welding and (b) laser welding. 2. Materials and Methods In the present work, the weld solidification cracking susceptibility of a Transformation-Induced Plasticity (TRIP) steel was studied using the aforementioned test arrangement. Crack length was measured for each case, and the fracture surface was observed by scanning electron microscopy. A Finite Element (FE)-based thermal elasto-plastic model of the welding was constructed to substantiate the results obtained from experiments. The occurrence of a crack was not simulated explicitly. Rather, the accumulated transverse strain along the direction perpendicular to the weld was used to assess the solidification cracking susceptibility. Based on the developed understanding, optimum processing conditions can be defined to weld steels that are otherwise susceptible to solidification cracking. 2.1. Experimental Section A 3 kW Nd:YAG laser was used as the heat source. The bead-on-plate laser welding experiments were conducted in keyhole mode using a power of 1100 W and a welding speed of 10 mm s−1. Specimens were kept at the focal point of the optical system, and the laser spot size was 0.6 mm. The experimental arrangement is shown schematically in Figure3. The coordinate system used in Metals 2018, 8, 673 4 of 15 the FE model is also included. The dimensions of the rectangular steel sheets were 90 × 45 mm2 and 1.25 mm thick. Table1 shows the elemental composition of the TRIP steel used in this work. For all the experiments, the starting beam position was at x = 0 mm, and welding was carried out along the x direction.